1. Field of the Invention
The present invention relates to a preparation process of polyhydroxycarboxylic acid which is useful as a biodegradable polymer that can be substituted for materials and general purpose resins.
2. Description of the Related Art
Polyhydroxycarboxylic acid is excellent in mechanical, physical and chemical properties and additionally has a biodegradable function so that it can be degraded in the natural environment without giving a harmful effect on living organisms and is finally decomposed by microorganisms into water and carbon dioxide. Thus, polyhydroxycarboxylic acid plastics have recently received particular attention in various fields such as alternatives for medical materials and general purpose resins.
As a common preparation process of polyhydroxycarboxylic acid, it has been known that a high molecular weight polymer can be obtained by conducting dehydration dimerization of hydroxycarboxylic acid, for example, lactic acid and glycolic acid, and successively subjecting the resultant cyclic dimer to ring-opening melt polymerization in the presence of a catalyst (Sn-base catalyst) (hereinafter referred to as an indirect polymerization process). In the process, however, reaction procedures are complex, the polymer obtained is expensive, and additionally, the shape of the polymer is restricted to pellets because the product of melt polymerization is pelletized as such. Further, the process cannot be applied to some of hydroxycarboxylic acid which do not form the cyclic dimer.
On the other hand, processes for obtaining polyhydroxycarboxylic acid by direct dehydration polymerization (hereinafter referred to as a direct polymerization process) have been disclosed in Japanese Laid-Open Patent SHO 59-096123 and 61-028521. These processes have no restriction mentioned above on the raw material and can prepare polymers and copolymers from various kinds of hydroxycarboxylic acid.
Further, the process is much simplified so as to obtain the polymer by the direct polymerization of hydroxycarboxylic acid and is thus a useful preparation process in industry.
Various processes have been known for separating polyhydroxycarboxylic acid from an organic solvent solution of the polymer. However, a satisfactory process in industry has not yet been known.
For example, a generally known process to obtain a solid polymer is to first dissolving the polymer in a good solvent and then depositing the polymer by bringing the polymer into contact with a non-solvent or a lean solvent. However, application of the method to the polymer leads to such problems that the deposited polymer forms a gel and stirring becomes difficult or the gel adheres to the inner wall of the vessel.
As a countermeasure for these problems, Japanese Laid-Open Patent SHO 63-254128 has disclosed a process for carrying out the purification and isolation of the polymer at the same time by bringing the polymer into contact with the depositing agent under the influence of a turbulent shear force so as to divide the deposited polymer into fine particles. The method, however, has a disadvantage that specific equipment must be used and thus an excessive plant cost is required.
Further, Japanese Laid-Open Patent SHO 58-206637 and 61-042531 have described a process for heat-dissolving polyhydroxycarboxylic acid in a solvent such as xylene or diethyl phthalate and removing the solvent by cooling the solution. The process can certainly provide powder of the polymer with ease. However, in the case of a polymer solution obtained by the direct polymerization process such as disclosed, for example, in Japanese Laid-Open Patent HEI 04-337321, a new solvent which differs from the solvent of polymerization is used. Moreover, when polyhydroxycarboxylic acid is deposited from the solvent according to the description, a large amount of the oligomer of the polymer remains in the separated solvent. Consequently, the separation and recovery of the solvent and oligomer become complex and require an excessive plant cost. Besides, the process has a problem that a high molecular weight polyhydroxycarboxylic acid solution having a high concentration solidifies the whole solution on cooling and thus the separation procedure of the polymer from the solvent by filtration according to the description is impossible to carry out.
The present inventors have obtained high molecular weight polyhydroxycarboxylic acid by carrying out direct polymerization of hydroxycarboxylic acid in a specific solvent and further found that the solid polymer can be obtained by controlling the reaction mass at a specific concentration and carrying out cooling crystallization (Japanese Laid-Open Patent HEI 06-172502). However, the form of the solid product obtained in the process can vary, such as in the form of powder, granules, block or mass depending upon the molecular weight of the polymer and the temperature condition at the crystallization step. Consequently, the concentration must be extremely lowered in the crystallization step in order to obtain the polymer in the form of powder or granule which can be handled with ease. Further, the above mentioned separation and recovery of the solvent and oligomer must be installed in order to separate the polymer from the solvent by filtration. Therefore, development of a technique for isolating the solid polyhydroxycarboxylic acid has been strongly desired in industry.